The price change reflects the end of a "promo" period, a T-Mobile spokeswoman said. The updated price only applies to new customers, so those who subscribed to One Plus while it cost $5 a month (or any earlier price) will not see any changes to their bill.

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Either way, the price hike makes T-Mobile's plan less competitive next to its peers. On its own, the One plan is less expensive than similar "unlimited" offerings from Verizon and AT&T, but limits all video streamed over its network to a 480p resolution, which is less than HD. It also does not let you access LTE data if you want to use your phone as a mobile internet hotspot; the plan caps you at slower 3G speeds instead.

To get either of those perks — as well as unlimited in-flight WiFi from Gogo and faster international data speeds — you have to sign up for One Plus.

Verizon and AT&T's unlimited plans do not have those restrictions by default: Both allow you to stream HD video (in AT&T's case, you have to enable it through an app first) and use 10 GB of LTE mobile-hotspot data without paying extra. Sprint does the same, though its network is generally seen as spottier than the other carriers.

Verizon's plan starts at $80 a month — the same as the revised One Plus plan — while AT&T's plan starts at $90 a month. Sprint's plan currently starts at $50 a month, though the company has said that is a promotional price that will end in the middle of next year.

It's worth noting, however, that T-Mobile's price includes the cost of additional taxes and fees, while the other three carriers' plans do not.

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And in every case, what carriers call "unlimited" is not truly unlimited: Beyond the caps on mobile-hotspot data, all four carriers say subscribers' speeds may be slowed if they use a certain amount of LTE data in a given month. In T-Mobile's case, that cap is roughly 32 GB.

T-Mobile has quietly altered the price of the One Plus add-on multiple times since introducing it last December. It originally cost an extra $15 a month. T-Mobile briefly included the HD video and LTE mobile-hotspot perks in the $70 One plan earlier this year, but put those back behind a paywall shortly thereafter.

It may not be the last time the One Plus add-on changes prices, either: The fine print on T-Mobile's website notes that the $10 monthly price is still a "limited-time offer."

T-Mobile has been the biggest winner in an increasingly competitive wireless market in recent years. Though it remains well behind Verizon and AT&T in terms of total subscribers, its user base has grown much faster as of late, largely on the back of an improved network and several aggressive promotions. The introduction of the One plan last year is largely seen as the catalyst for the recent revival of "unlimited" data plans at large.

Past rumors have tied it to a potential merger with Sprint, though recent reports have said that possibility is currently on hold.

But with the price hike here, T-Mobile's status as a good value is a little cloudier.